I used Tir as part of the first prototype before writing the
proposal. This was in May, but afterward we decided to opt for full
client-side implementation for the web UI, whereas Tir seems to target
server-side web applications. Even for those, the value add was rather
thin, which is quite natural because the author advertises to use only
1300 lines of code.

The proposal mentions Backbone.js, but after a second prototyping
round I feel that this library creates more problems than provides
solutions, at least in this context. Backbone's data model is rather
limited in comparison to what is proposed for the new ACF. For
example, Backbone does not support nested collections or collections
of primitive types. Circumventing these limitations provided to add
too much complexity both to the server and client implementation.

As much as I dislike JavaScript as a programming language, I still
think it is the best choice for client implementation. Virtually all
devices support JavaScript nowadays, and there are lots of tools which
we can leverage in our implementation. The selection of Lua tools in
this domain is rather modest in comparison, so the KISS principle is
probably better realized with JavaScript.